The goal of this project is to determine the nature of the cognitive mechanisms that are disrupted in the various types of number processing and calculation deficits (dyscalculia) found in patients with focal brain damage. This issue will be addressed through a two-phase program of research. In the first phase a large number of patients with focal lesions (cerebro-vascular accidents) will be tested with a number processing/calculation test battery. The test battery is designed to distinguish deficits in number comprehension and/or production from calculation deficits, and to draw further distinctions within each of these two major classes. For example, within the calculation deficits, distinctions can be made between impairments in recall of basic arithmetic facts (e.g., 4x9=36), and deficits in execution of calculation procedures. The battery results will be used to define subgroups of patients with selective deficits involving various basic components of the number processing/calculation cognitive systems (e.g., arithmetic fact retrieval deficit) and to evaluate a hypothesized model of number processing and calculation. In the second phase of the research the subgroups defined on the basis of test bettery results will be studied extensively using a combined group/case study methodology. The patients in each group will be tested extensively with experimental tasks designed to elucidate the nature of the cognitive mechanisms within each major number processing component, and the ways in which these mechanisms break down as a consequence of brain damage. Information generated from the detailed analyses in this phase of the research will also be used to revise and refine the battery to better discriminate among different patterns of dyscalculia in different types of neurologically impaired patients.